This invention relates to a television antenna. More particularly, this invention relates to a television antenna of the dipole type.
An efficient set-top television antenna must be designed while keeping in mind many considerations in practical electromagnetics. Dipole antennas for receiving commercial television broadcasts must satisfy or balance among many inharmonious engineering requirements. For example, an all-channel antenna must be capable of adequately receiving signals across a broad spectrum of frequency ranges which extend from a band of 54 MHz to 72 MHz, in 6 MHz increments (Channels 2, 3, and 4), to a UHF band of 470 MHz to 890 MHz, in 6 MHz increments (Channels 14 through 83). For each of the lowest numbered channels, because the channel bandwidth exceeds 10 percent and wavelength exceeds 16 feet, dipole antennas sized for set-top installation exhibit such highly capacitative and high Q impedance properties that individual channel matching circuits are needed to increase the amount of broadcast signal delivered to the receiver. For the highest numbered channels, because bandwidth is less than one percent and wavelength is less than 14 inches, antennas sized for set-top installation can exhibit inherently satisfactory impedance properties over a frequency range encompassing many channels, often the entire UHF broadcast band.
A basic problem in attempting a set-top design that receives all VHF and UHF broadcast channels efficiently is that an antenna with an electrical extent large enough to facilitate impedance matching at each of the lowest frequency channels, particularly Channels 2 and 3, has such a large electrical extent at the highest frequency channels that the radiation patterns can exhibit sizeable frequency variations, leading to amplitude and/or phase weightings of a received broadcast signal over its frequency band and substantially diminished quality of reception. Additionally, the small electrical extent of a set-top dipole antenna at each of the lower frequency channels ensures a low value of radiation resistance while, at the higher VHF channels, its electrical extent can approach a wavelength, ensuring a high value of radiation resistance. In summary, the antenna's electrical extent is too little at low VHF because of low resistance levels, too large at high VHF because of the high resistance levels, and much too large at UHF because of pattern dispersions. The superposition of an antenna configuration that can perform well at Channel 2 with one that can perform well at Channels 3, 4, etc., and can be easily reconfigured for efficient operation at any channel is a demanding requirement because of the inharmonious factors and the resulting inharmonious engineering requirements.
A basic problem in set-top antenna design arising from these inharmonious engineering requirements is that the antenna's electrical extent is generally too little in one band and too large in another. Because of the large wavelengths and appreciable percentage bandwidths required in the lowest frequency channels (2 and 3), efficient reception in these channels, which depends on the antenna's size in wavelengths, conflicts with its height, its width and the length of its dipole arms. The super-position of an antenna configuration that can perform well at Channel 2 with one that can perform well at Channels 3 and 4, etc. and can be easily reconfigured for efficient operation at any channel is a demanding requirement.
In dipole antenna assemblies, the antenna is connected to a transformer via a balanced two-wire transmission line. The transformer is in turn connected to the television antenna input port via an unbalanced coaxial transmission line. The devices for transforming from balanced to unbalanced transmission lines (baluns) must be very large to operate efficently at the lower frequencies or must employ ferrite devices that result in attenuation at the higher frequencies. Similarly, some of the circuit components used in "broadband" matching over the extent of each channel at lower frequencies can be sizable or can use ferrite devices with attenuation at lower frequencies.
Improving the reception range of indoor, set-top television antennas is limited by the available height above ground and the achievable antenna gain, particularly at the low frequency channels. Actually, given the size limitations that must be imposed on set-top antennas, the only feasible improvements are in the efficiency of the antenna in extracting the maximum broadcast signal from the air and in delivering that signal to the television receiver.
It has been discovered that most fully extended rabbit-ear type antennas are at least 10 dB deficient at Channel 2 compared with a reference high quality dipole configured for a frequency of 57 MHz. It is likely that a major cause of this low efficiency is radio-frequency attenuations between interconnected sections of the telescoping tubes of the antenna. It is also possible that these losses increase with time and use.
It appears that the conventional rabbit-ear antenna is in need of replacement with an updated or improved design.